There is a major need for a system which will automatically expose and handle double-sided printed circuit boards, especially such boards having large areas. For example, each board may be 18.times.24 inches in size, although much larger sizes may also be handled. The "boards" may be either relatively rigid (with typical thicknesses in the range of 0.04 inch to 0.1 inch), or they may be multilayer boards as thin as 0.005 inch.
Each side of each board is made sensitive to ultraviolet light, so that it may receive an image corresponding to that of artwork, there being one artwork (actually, a photomask) for one side and another artwork for the other side. It is extremely important that there be precise registry between the artwork and the board, so that the resulting images produced by masking and exposure will register on opposite sides of the board. Then, connections may be made through the board to registered portions of the electric circuits.
The artwork is often extremely fine, so as to be difficult or even impossible to see with the naked eye. For this and numerous other reasons, the boards cannot be scratched or scuffed, since scratching (typically caused by scuffing) creates expensive rejects, incapable of satisfactory commercial operation.
During exposure, the artwork and the adjacent side of the board are caused to be in proximity or in surface engagement with each other. The maximum "proximity gap" typically permitted is 0.04 inch, and it must be uniform. In other words, it is not satisfactory for certain portions of the board to be extremely close to the artwork or touching it, while other portions are much less close to the artwork.
It was conventional in the prior art to effect exposure of double-sided large-area circuit boards manually, using clumsy and cumbersome apparatus. For example, a "sandwich" made up of two layers of artwork, a circuit board, supporting apparatus, and flexible transparent material on which a vacuum was drawn, was placed on a tray and slid into a machine that exposed simultaneously the top and bottom surfaces of the board. (It is to be understood that the boards of prior-art and the present system have light-sensitive dry film cover sheets or have liquid resist coated onto the surfaces.) The indicated apparatus required the full-time presence of an operator and could not effectively be automated into assembly lines containing apparatus employed in other steps necessary for the manufacture of printed circuit boards.
The present system (apparatus and method) provides automatic, relatively high-speed, scuff-proof exposure of both sides of printed circuit boards. The system is located in-line with other desired apparatus, and at least major portions thereof are preferably enclosed in a housing (not shown). The boards go in one end and come out the other with both sides properly exposed and correctly registered.